Thursday, November 30, 2023
| Demographic | Age-standardized average incidence, 1992–2002 | Age-standardized incidence, 2018 | Absolute increase in age-standardized incidence | Age-standardized incidence risk ratio, 2018 to 1992–2002 baseline (95% CI) | Increase in age-standardized incidence, % |
|---|---|---|---|---|---|
| Age group, y, not standardized | |||||
| 0–4 | 0.03 | 0.01 | −0.03 | 0.16 (0.02–1.19) | −83.52 |
| 5–14 | 0.02 | 0.01 | −0.01 | 0.48 (0.15–1.54) | −51.61 |
| 15–24 | 0.07 | 0.19 | 0.12 | 2.80 (2.19–3.57) | 179.80 |
| 25–34 | 0.18 | 0.75 | 0.58 | 4.30 (3.79–4.88) | 330.31 |
| 35–44 | 0.38 | 1.97 | 1.59 | 5.15 (4.74–5.59) | 414.89 |
| 45–54 | 0.66 | 4.12 | 3.46 | 6.28 (5.91–6.69) | 528.44 |
| 55–64 | 1.02 | 6.52 | 5.50 | 6.39 (6.05–6.75) | 539.14 |
| 65–74 | 1.42 | 7.66 | 6.24 | 5.40 (5.11–5.70) | 439.63 |
| 75–84 | 1.57 | 8.52 | 6.96 | 5.44 (5.07–5.84) | 444.13 |
| ≥85 | 1.49 | 9.69 | 8.20 | 6.50 (5.82–7.27) | 550.35 |
| Sex | |||||
| M | 0.63 | 3.66 | 3.04 | 5.86 (5.67–6.05) | 485.55 |
| F | 0.35 | 1.86 | 1.50 | 5.29 (5.06–5.53) | 429.22 |
| Race* | |||||
| Native American or Alaska Native | 0.26 | 1.27 | 1.01 | 4.93 (3.51–6.93) | 392.94 |
| Asian or Pacific Islander | 0.14 | 0.56 | 0.42 | 4.03 (3.19–5.10) | 303.18 |
| Black or African American | 0.47 | 5.21 | 4.74 | 11.04 (10.39–11.73) | 1003.95 |
| White | 0.37 | 1.99 | 1.61 | 5.30 (5.12–5.49) | 430.15 |
| Region | |||||
| Division | |||||
| Northeast | 0.68 | 4.82 | 4.14 | 7.04 (6.70–7.40) | 604.10 |
| New England | 0.61 | 4.33 | 3.72 | 7.10 (6.40–7.87) | 610.04 |
| Middle Atlantic | 0.71 | 5.00 | 4.30 | 7.07 (6.69–7.48) | 606.98 |
| South | 0.33 | 1.97 | 1.64 | 5.97 (5.67–6.29) | 497.23 |
| South Atlantic | 0.44 | 2.29 | 1.85 | 5.24 (4.91–5.59) | 423.54 |
| East South Central | 0.32 | 2.05 | 1.73 | 6.40 (5.63–7.27) | 539.66 |
| West South Central | 0.15 | 1.36 | 1.21 | 9.15 (8.10–10.34) | 815.03 |
| Midwest | 0.67 | 4.10 | 3.43 | 6.13 (5.85–6.42) | 513.06 |
| East North Central | 0.77 | 5.01 | 4.24 | 6.48 (6.16–6.82) | 548.02 |
| West North Central | 0.42 | 2.04 | 1.62 | 4.81 (4.29–5.40) | 381.38 |
| West | 0.29 | 0.99 | 0.70 | 3.39 (3.11–3.68) | 238.50 |
| Mountain | 0.43 | 1.07 | 0.64 | 2.47 (2.15–2.83) | 146.55 |
| Pacific | 0.23 | 0.95 | 0.72 | 4.13 (3.71–4.59) | 312.91 |
| United States | 0.48 | 2.71 | 2.23 | 5.67 (5.52–5.83) | 467.30 |
Table. Magnitude of increase in age-standardized incidence of Legionnaires’ disease, cases/100,000 population, from 1992–2002 (average) through 2018, United States
*Ethnicity was not analyzed because data were missing for 30.4% of cases.
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During 1992–2002, an average of 1,221 (range 1,060–1,547) LD cases were reported annually; during 2003–2018, an average of 4,369 (range 2,082–9,999) cases were reported annually. Crude and age-standardized incidence increased from 0.52 and 0.55 cases/100,000 population in 1992 to 3.06 and 2.71 cases/100,000 population in 2018 (Figure 2). Over the study period, joinpoint analysis selected a model with 1 change point in the trend in age-standardized incidence as the best model (over models with zero or 2 change points). Although joinpoint analysis identified the single optimal change point in the trend in age-standardized incidence (p<0.05) as 1999 (95% CI 1996–2002), we retained 2002 as the breakpoint in our analyses to aid in comparisons with previous studies. In addition, the largest relative increase (26%) in a 3-year moving average of age-standardized incidence over the study period occurred in 2003. From 1992 to 2002, no indication of a trend in age-standardized incidence was seen (−0.2%, 95% CI −5.1% to 5.0%); from 2002 to 2018, the average annual increase in age-standardized incidence was 9.3% (95% CI 8.1%–10.4%), of which the largest increase occurred during 2016–2018.
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EnlargeFigure 2. Reported cases of Legionnaires’ disease by month and incidence (cases/100,000 population) by year, United States,1992–2018. Monthly cases reported to the Centers for Disease Control and Prevention through the National Notifiable Diseases Surveillance System and the crude and age-standardized annual incidence for 1992–2018 are shown.
Age data were available for 82,649 (99.2%) of the 83,334 cases in the study period. During the baseline years, the largest number of average annual cases (257) was reported in the 65–74-year age group; the average number of cases in the 2 older age groups (75–84 and ≥85 years) was lower than the 2 younger age groups (45–54 and 55–64 years) (Figure 3, panel A). Average age-specific incidence generally increased with age, rising from <0.1 cases/100,000 population in children and young adults (0–24 years) to peak in the 75–84-year age group (1.57 cases/100,000 population) (Appendix Table). During the increase years, the largest number of average annual cases (1,122) was reported in the 55–64-year age group, and the distribution was more symmetric around this peak (Figure 3, panel B) than around the peak for the baseline years. Except for the 0–14-year group, in which incidence remained low (<0.1 cases/100,000 population), average age-specific incidence increased with age through the ≥85 years category (5.52 cases/100,000 population).
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EnlargeFigure 3. Average annual number of cases of Legionnaires’ disease and average incidence (cases/100,000 population), by age group, United States, 1992–2018. A) Reported average number of annual cases and average incidence by age group for 1992–2002. B) Reported average number of annual cases and average incidence by age group for 2003–2018.
Joinpoint analysis identified 2002 as the change point in the trend of median patient age (Figure 4). Median patient age decreased from 62 years in 1992 to 58 years in 2002, then increased to 62 years in 2018. We identified a model with no change points as the best model for the trend in mean patient age over the study period; mean age increased from 58.9 years to 61.7 years.
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EnlargeFigure 4. Trends in median and mean age of Legionnaires’ disease patients by year, United States, 1992–2018.
During 1992–2002, men accounted for 59.8% of the 13,137 cases for whom sex and age were reported, compared with 62.8% of 69,226 cases during 2003–2018. The age-standardized average incidence in men was 0.63/100,000 men and in women was 0.35/100,000 women during 1992–2002 (Appendix Table). During 2003–2018, the age-standardized average incidence increased to 1.80/100,000 in men and 0.91/100,000 in women.
Race or age was missing for 16.5% of cases; thus, race-specific case counts and incidences might be slightly higher than measured in this study. During the baseline years, >6 times the number of average annual cases were reported among White persons (813) than Black or African American persons (128), but the age-standardized average incidence was >25% higher among Black or African American persons (0.47/100,000 population) than White persons (0.37/100,000 population) (Figure 5, panel A; Appendix Table). This pattern continued, and racial disparities were more pronounced during the years of increase, when the age-standardized average incidence was twice as high among Black or African American persons (2.15/100,000 population) than among White persons (0.99/100,000 population) (Figure 5, panel B).
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EnlargeFigure 5. Average annual number of cases of Legionnaires’ disease and age-standardized average incidence (cases/100,000 population) by race, United States, 1992–2018. A) Reported average number of annual cases and age-standardized average incidence by race for 1992–2002. B) Reported average number of annual cases and age-standardized average incidence by race for 2003–2018.
During both the baseline years and the years of increase, the age-standardized average incidence was higher in the Northeast (0.68/100,000 population in baseline years; 2.34/100,000 population in years of increase) and Midwest (0.67; 1.67) regions than in the South (0.33; 1.01) and West (0.29; 0.66) regions (Appendix Table). Similarly, the contiguous East North Central (0.77; 2.01), Middle Atlantic (0.71; 2.59), and New England (0.61; 1.64) divisions had the highest age-standardized average incidence during the baseline years and the years of increase. Among the 20 jurisdictions with the highest age-standardized average incidence during 1992–2002, a total of 10 were located within the East North Central, Middle Atlantic, or New England divisions, and 3 others bordered these divisions (Figure 6, panel A). During 2003–2018, 14/20 jurisdictions with the highest age-standardized average incidence were located within these same 3 divisions, and 4 additional jurisdictions (of the 20) bordered these divisions (Figure 6, panel B).
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EnlargeFigure 6. Age-standardized average incidence (cases/100,000 population) of Legionnaires’ disease by jurisdiction, United States, 1992–2018. A) Age-standardized average incidence by jurisdiction, 1992–2002. Legionnaires’ disease was not reportable in Connecticut during 1992–1996 or in Oregon or West Virginia during 1992–2002. B) Age-standardized average incidence by jurisdiction, 2003–2018.
Most LD cases occurred during summer or fall months, and this pattern became more extreme after the baseline years (Figure 2). During 1992–2002, an average of 57.8% of annual cases occurred during June–November, increasing to 68.9% during 2003–2018. The average annual maximum-to-minimum monthly cases ratio rose from 2.59 during the baseline years to 4.31 during the years of increase.
By geography, during the baseline years, moderate seasonality was observed in the Northeast region and less so in the Midwest and South regions (Figure 7, panel A). No seasonal pattern was discernible in the West. When cases increased during 2003–2018, seasonality became more prominent in all regions, particularly in the Northeast and Midwest (Figure 7, panel B). A less pronounced but identifiable seasonal pattern was also observed in the West. The LD season began first in the South and maintained a peak in this region from June through October. The LD season began later in the Midwest and Northeast, peaking in July in the Midwest and in August in the Northeast.
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EnlargeFigure 7. Seasonality of Legionnaires’ disease cases by Census Bureau region, United States, 1992–2018. A) Seasonality of cases by US Census Bureau region, 1992–2002. The monthly percentage of each region’s cases is shown. If no seasonality existed, approximately the same number of cases would be expected to occur each month (i.e., 1/12 [8.3%] of annual cases would occur each month). B) Seasonality of cases by US Census Bureau region, 2003–2018. The monthly percentage of each region’s cases is shown. If no seasonality existed, approximately the same number of cases would be expected to occur each month (i.e., 1/12 [8.3%] of annual cases would occur each month).
Overall, age-standardized average incidence increased from 0.48/100,000 population during the baseline years (1992–2002) to 2.71/100,000 population in 2018 (incidence risk ratio [RR] 5.67, 95% CI 5.52–5.83) (Table ). Relative changes in incidence in the 0–4-year and 5–14-year age groups were not statistically significant (RR 0.16, 95% CI 0.02–1.19 for 0–4 years; RR 0.48, 95% CI 0.15–1.54 for 5–14 years). Incidence increased >5-fold for all age groups above 34 years; the largest relative increases occurred in the ≥85-year (RR 6.50, 95% CI 5.82–7.27), 55–64-year (RR 6.39, 95% CI 6.05–6.75), and 45–54-year (RR 6.28, 95% CI 5.91–6.69) age groups. Age-standardized incidence increased slightly more in men (RR 5.86, 95% CI 5.67–6.05) than in women (RR 5.29, 95% CI 5.06–5.53). The age-standardized incidence increased from 0.47 to 5.21/100,000 population in Black or African American persons (RR 11.04, 95% CI 10.39–11.73) and from 0.37 to 1.99/100,000 population in White persons (RR 5.30, 95% CI 5.12–5.49).
By region, the relative increase in age-standardized incidence was largest in the Northeast (RR 7.04, 95% CI 6.70–7.40), similar in the Midwest (RR 6.13, 95% CI 5.85–6.42) and South (RR 5.97, 95% CI 5.67–6.29), and smallest in the West (RR 3.39, 95% CI 3.11–3.68). By division, the largest relative increase in age-standardized incidence occurred in the West South Central division (RR 9.15, 95% CI 8.10–10.34). The next-largest relative increases were similar among the New England (RR 7.10, 95% CI 6.40–7.87), Middle Atlantic (RR 7.07, 95% CI 6.69–7.48), East North Central (RR 6.48, 95% CI 6.16–6.82), and East South Central (RR 6.40, 95% CI 5.63–7.27) divisions. The smallest relative increase in age-standardized incidence was in the Mountain division (RR 2.47, 95% CI 2.15–2.83). Although the largest relative increase in age-standardized incidence occurred in the West South Central division, the largest absolute increases occurred in the Middle Atlantic, East North Central, and New England divisions.
